Next door a display marks – not celebrates, said NPG associate curator Rosie Broadley – a significant moment in the gallery's history: the day, 100 years ago this month, that a suffragette attacked one of its paintings.

Broadley said for a long time the only portraits the gallery had of suffragettes were the surveillance photos supplied to it by the police.

The painting of Christabel Pankhurst shows a woman in full swagger, wearing her suffragette sash with pride and emerging from darkness looking as if she is about to speak with passion.

"It is an amazing painting, a historical document as much as portrait," said Broadley. "It was supposed to inspire people and present Pankhurst as a figurehead, leading a charge."

Pankhurst was the daughter of the suffragette leader Emmeline Pankhurst and a prominent strategist in the campaign. By 1912, fed up with the limited success of moderate tactics, she was urging a more militant approach.

It shows the increasing attacks on works of art and the Edwardian police photographs of suffragettes suspected of dastardly deeds, which were given to the NPG by the Criminal Record Office.

Police surveillance photographs of militant suffragettes – for a long time the only portraits the NPG had of the women. Photograph: National Portrait Gallery London

One of the highest profile attacks was by the chopper-armed Mary Richardson on the Rokeby Venus by Velázquez at the National Gallery.

Fears of further attacks led to women visiting the gallery having to leave their muffs and parcels in the cloakroom for fear of concealed weapons.

The British Museum went a step further: only allowing women in if they were accompanied by a man who would take responsibility for their actions.

After Emmeline Pankhurst was arrested yet again in July 1914, Anne Hunt, real name Margaret Gibb, entered the NPG with a butcher's cleaver and ripped through Millais's portrait of Thomas Carlyle, the gallery's founder.

Hunt got the maximum sentence, six months, but was released quite quickly and an internal document on display reveals that she tried to re-enter the NPG on 31 August but was spotted and attendants told to bar her in future.

The full-length portrait of Christabel Pankhurst was painted in 1909 by Ethel Wright, a society portraitist who supported the suffrage cause.

It was bought by the prominent suffragette Una Dugdale Duval, a woman who sparked incredulity in polite society by refusing to promise to "obey" at her wedding. The portrait passed down through the family and was bequeathed to a grateful NPG in 2011.